1. Field of the Invention
The present invention relates to an inkjet recording method of using comparative high viscous inks which are hardened, for example, by ultraviolet rays. Particularly, this invention relates to an inkjet recording method which has excellent ink-ejection stability and outputs high-definition images with suppressing generation of ink mists, and to inkjet recording inks for the method.
2. Background of the Art
Various researches and developments have been made on inkjet recording methods to improve and give higher functions to inkjet recording inks (hereinafter simply called “inks”) and inkjet nozzles.
Recently, various kinds of inkjet recording methods have been available as solvent inkjet printers which use particular solvents, UV-setting inkjet printers which do not choose recording media, and so on. They have been vigorously applied to printing in graphic fields, printing of flat-panel displays (organic EL polymer, organic EL light emitter, liquid crystal spacer, liquid crystal color filter, etc.), and printing of substrates.
Various kinds of ink have been used such as inks which use pigment, dispersed dye, metal particles, and so on as color materials; inks whose physical property greatly changes by gelation or exposure to ultraviolet rays after being ejected; and inks which use high-security solvents. These highly functional inks are apt to lose their ejection abilities quickly. This is mainly because the additives to give functionalities to the ink increase the viscosities of the ink and thixotropic properties in the dispersion system, and reduces the surface tension of the ink liquid.
Meanwhile, the recording head of the inkjet printing apparatus has been requested to accomplish downsizing of ink droplets, increase of gradations, increase of production volumes, increase of the number of nozzles, higher ink ejection accuracy, and durability and reliability of functional ink.
The use of high-viscous dispersed inks and small droplets and high frequency head in combination may be a problem in assuring the stability of ink ejection. Particularly, major problems in high-frequency ejection of high viscosity inks through nozzles of a small diameter are generation of many satellites from ink droplets and resulting ink mists which waft in the air before the recording medium.
To make each droplet land exactly, the speed of a main droplet must be increased to some extent. A higher liquid speed makes the liquid column from a nozzle longer and is apt to generate ink satellites. If the ink has a low surface tension, the liquid column is broken into a lot of small liquid pieces (satellites) of lower speeds. This becomes an ink mist.
This ink mist not only contaminates the recording medium but also deteriorates the ink ejection ability when deposited on the inkjet nozzle. Further, to remove ink deposit and stabilize ink ejection, an additional process is required to clean the inkjet nozzle. This will reduce the print productivity of the inkjet recording apparatus and make the apparatus complicated (by provision of a recovery mechanism). If the relative speed between the carriage and the recording medium is increased to improve the print productivity, the satellites may be affected by winds and lose their speed. This will increase the ink mist. This phenomenon becomes more striking when main droplets are made smaller.
If the ink contains a persistent material such as pigment, persistent polymer, inorganic material, or metal, a mist of micro-particles (e.g. some μm or particularly 1 μm or less in size) of such persistent inks is harmful to human bodies because, when inhaled deep in the alveoli of the lung, the micro-particles will remain there.
Some methods have been proposed to suppress generation of ink mists. They are a method of suppressing disturbance of the air caused by the reciprocal movement of the carriage (see Patent Document 1), a method of attracting the ink mist by an electrostatic member (see Patent Document 2), a method of attaching the satellites by a certain driving condition of the piezoelectric element to the main droplets (see Patent Document 3), and a method of coalescing satellites into larger droplets before they reach the recording medium (see Patent Document 4).
However, the above methods are not enough to substantially suppress contamination of recording media with ink, ink deposition on inkjet nozzles, and deterioration in ink ejection stability. The generation of ink mists is dependent upon ink prescriptions, head driving conditions, and recording conditions in addition to the conditions of small droplets (20 pl or less), multi-gradation recording by multi-drops, high-speed movement of inkjet nozzles to get high print productivity (carriage speed), use of functional pigments, and high-viscosity ink which mainly contain polymeric monomers. Therefore, it has been very difficult to dissolve such problems.
Particularly, it has been very difficult for process color inks to have good dispersibilities. Especially, yellow and magenta pigments are apt to generate ink mists.
[Patent Document 1]
Japanese Non-Examined Patent Publication 2004-42580
[Patent Document 2]
Japanese Non-Examined Patent Publication 2003-237110
[Patent Document 3]
Japanese Examined Patent Publication H05-57913
[Patent Document 4]
Japanese Non-Examined Patent Publication 2002-144570